I think at some point any piece of software requires a major rewrite (of course, when that point is depends). Whenever software is written, fundamental assumptions get made, and even the most forward thinking architects can't predict everything that will become possible or necessary. For a while, it is simply enough to build on top of what you currently have, but eventually the underlying architecture will no longer represent the main goals of the app. At this point, the spaghetti and hacks will far outnumber the well written aspects of an application and you will be spending most of your time trying to navigate the mess.

Again, that's not to say all software should be rewritten for every version. Mac OS X has been around for about 10 years and could easily go for another 10 with its current architecture. However, I think we can all agree that sometime between now and 100 years from now, something fundamental will probably have to change.